Internal combustion engines and static hydrocarbon burning equipment tend to emit, via their exhaust systems, carbonaceous particles commonly referred to as particulates. Whilst unrelenting efforts are being expended towards reducing particulate emissions at source, particulate filters (traps) in the exhaust systems of such equipment are becoming essential to meet increasingly strict environmental legislation and public expectations.
Particulate filters which may be regenerated are known. In some cases these require regular removal from the equipment to which they are fitted followed by burning (oxidation) of the trapped particulates and refitment to the equipment. Particulate filters may be regenerated in this way by removal at the end of a working day, heating to a high temperature overnight to burn the collected particulates and refitting in the morning.
The removal, cleaning and refitting process has the disadvantage that the equipment is taken out of service for several hours, labor is required to remove, clean and refit the filter and the filter is generally subjected to a set cleaning process irrespective of the level of particulate build-up within it. Further, as the filter builds up its particulate content in use, resistance to exhaust gas flow is increased and thus an oversized filter may be required.
Taking mobile on-highway and off-highway vehicles out of service to regenerate the filter is particularly undesirable, and regeneration in situ offers clear advantages.
A conventional option is to provide two parallel filters, each one of which is alternately by-passed for regeneration in situ. However, there is a significant size and cost penalty with this system, the size of a particulate filter being typically the same as the swept volume of an associated naturally-aspirated engine and up to three times the swept volume of an associated turbocharged engine.
It is clearly desirable for regeneration to occur during use. However, for rapid oxidation of trapped particulate there must be sufficient free oxygen available, preferably included within the exhaust stream at its source. The free oxygen within the exhaust stream from a diesel engine ranges from 3% to 20%. In addition, unless pre-treated by, for example, catalyst or fuel additive means, the particulate must be at a temperature of at least 550° C. for rapid oxidation to take place. However, this level of exhaust gas temperature occurs only for the upper part of an engine load-speed map, therefore extra energy must be put into the exhaust gas stream, the filter or the trapped particulate itself for it to be raised above 550° C. for all engine operating conditions.
A conventional form of regenerating particulate filter incorporates a fuel burner system. This relies rely upon large amounts of heat being introduced into the exhaust system by fuelling, thus low particulate loadings can be oxidized. However, there is an unreliability concern with fuel burner systems associated with the required burner, ignition means, air pumps and isolation valves and the cost of such systems is relatively high. Further, most conventional systems require a method of measuring the amount of trapped particulate before regeneration commences, this being difficult and usually too inaccurate for optimum reliability of operation. Also, such a system will inherently have a detrimental effect on fuel consumption.
Other systems have been suggested which rely upon a control of the engine throttling. In essence, such systems have a throttle arrangement that is used intermittently to cause the engine to run richer at certain times to promote higher exhaust temperatures. Such systems have proven to be noisy in practice and also to have an adverse effect on both vehicle driveability and fuel economy.
Systems which use electrical resistance heaters to impart the extra energy have been proposed. However conventional electrical resistance heaters are likely to require significant energy input which can place an unacceptably high load on the engine's battery/alternator system.
There have been suggestions that the use of microwave energy to impart the extra energy required might overcome some of these difficulties. However, such systems may have safety implications, and are not necessarily cheap.
Furthermore, reliability problems may arise with many conventional regenerating particulate filter systems as a result of their reliance on the exothermic nature of the oxidation to sustain the regeneration reaction. This leads to a basic requirement to have an optimum amount of trapped particulate in the filter to promote efficient oxidation. If there is too much particulate, then the heat released during oxidation may cause melting of the filter element or monolith leading to an almost total loss of filtration. If there is too little trapped particulate, there may be insufficient chemical energy to maintain oxidation and the process may die out leaving an unregenerated filter. Fuel burner systems depend less on the chemical energy in the particulate to sustain oxidation, but more on putting large amounts of heat energy into the exhaust stream. However, although this allows potentially lower particulate loadings to be oxidized, the unreliability of the fuel burner systems associated with the burner, ignition means, air pumps and isolation valves has already been outlined.
A known apparatus directed to avoid this operates by introducing fuel additives in controlled doses to reduce the temperature required to burn off particulates in conjunction with engine control strategies such as pilot injection via a common rail injection system to elevate the exhaust temperature, plus oxidation and reduced-temperature catalysts and a sensor for detecting when regeneration is needed. This system thus relies on an amalgamation of potentially complex and expensive equipment.
It will be seen from the above commentary that it is desirable for a particulate filter to be self-regenerating in use, under any load, in order to maintain filtering and gas-flow efficiencies above a certain level whilst keeping filter sizing to a minimum. It is also desirable that the filter is self-controlled to regenerate only when a predetermined level of particulates is present and to do so without requiring any external sensing means. It is further desirable that the regeneration process is economic in the use of any externally supplied energy or material, that the construction of the filter is also economic, and that the system is effective irrespective of types and compositions of fuel and engine operating conditions.
WO 94/07008 discloses an apparatus and a method said to provide a self-controlled, self-regenerating, particulate filter apparatus in which electrical spark and/or short time duration (“preferably between 0.001 sec and 0.1 sec”) arc discharges oxidize and burn trapped particulates. It is asserted within '07008 that the electrical discharges will occur only when carbon has accumulated to a sufficient thickness and homogeneity to become electrically conducting, which leads to a spark and/or arc discharge between the conducting layer and the electrodes when a certain limiting layer is reached.
In order to function, the apparatus would require at least a heavily loaded filter and it would not be expected to work at all at low loadings. Further, the preferred frequency of 50 Hz is within the human aural range and in a range which is not easily muffled, therefore audible noise emissions may result.
WO 94/07008 also recites that it is particularly suitable to divide the gas stream into two different streams, with a different filter being installed in each, and that it is particularly advantageous to provide the addition of air and/or oxygen to the gas stream during regeneration. It is clear that each of these operations will incur the need for additional apparatus which will add to the cost and size of the overall system.
WO 94/07008 describes simple filter tube or plate arrangements rather than conventional particulate filter monoliths. These simple arrangements present only a limited filter surface area for given size and, would need to be impracticably large in order to present a filtering surface of sufficient area to avoid unacceptable back-pressure. In contrast, a conventional particulate filter monolith may typically comprise a cylinder of 250 mm length and 150 mm diameter enclosing 2800 longitudinal cells, the walls of 1400 of which provide a very large filter surface area. It would not be possible to incorporate such a conventional monolith into the regenerating apparatus of '07008.